Electrochemical degradation of PFOA and its common alternatives: Assessment of key parameters, roles of active species, and transformation pathway

Abstract

This study investigates an electrochemical approach for the treatment of water polluted with per- and poly-fluoroalkyl substances (PFAS), looking at the impact of different variables, contributions from generated radicals, and degradability of different structures of PFAS. Results obtained from a central composite design (CCD) showed the importance of mass transfer, related to the stirring speed, and the amount of charge passed through the electrodes, related to the current density on decomposition rate of PFOA. The CCD informed optimized operating conditions which we then used to study the impact of solution conditions. Acidic condition, high temperature, and low initial concentration of PFOA accelerated the degradation kinetic, while DO had a negligible effect. The impact of electrolyte concentration depended on the initial concentration of PFOA. At low initial PFOA dosage (0.2 mg L−1), the rate constant increased considerably from 0.079 ± 0.001 to 0.259 ± 0.019 min−1 when sulfate increased from 0.1% to 10%, likely due to the production of SO4•–. However, at higher initial PFOA dosage (20 mg L−1), the rate constant decreased slightly from 0.019 ± 0.001 to 0.015 ± 0.000 min−1, possibly due to the occupation of active anode sites by excess amount of sulfate. SO4•– and •OH played important roles in decomposition and defluorination of PFOA, respectively. PFOA oxidation was initiated by one electron transfer to the anode or SO4•–, undergoing Kolbe decarboxylation where yielded perfluoroalkyl radical followed three reaction pathways with •OH, O2 and/or H2O. PFAS electrooxidation depended on the chemical structures where the decomposition rate constants (min−1) were in the order of 6:2 FTCA (0.031) > PFOA (0.019) > GenX (0.013) > PFBA (0.008). PFBA with a shorter chain length and GenX with –CF3 branching had slower decomposition than PFOA. While presence of C–H bonds makes 6:2 FTCA susceptible to the attack of •OH accelerating its decomposition kinetic. Conducting experiments in mixed solution of all studied PFAS and in natural water showed that the co-presence of PFAS and other water constituents (organic and inorganic matters) had adverse effects on PFAS decomposition efficiency.